1. Field of the Invention
The present invention relates to a nonreciprocal circuit device such as an isolator or a circulator used in a high-frequency band such as the microwave band, and to a communication device using the same.
2. Description of the Related Art
In recent years, with mobile communication devices speeding toward miniaturization, a further reduction in the size of nonreciprocal circuit devices used in these devices is strongly required. In order to realize this size-reduction, Japanese Unexamined Patent Application Publication No. 11-97908 has disclosed a nonreciprocal circuit device using a magnetic body having a rectangular parallelepiped shape. When using such a magnetic body having a rectangular parallelepiped shape, one center conductor is disposed parallel with one side of the magnetic body, the two other center conductors are disposed so as to tilt toward each side, and all center conductors are arranged so as to intersect one another at an angle of substantially 120 degrees, in a state of being electrically insulated from one another. Herein, the conductor widths and the inter-conductor spacings of these center conductors used are each set to be the same.
Typically, the impedance of an input/output portion in the circuit where these nonreciprocal circuit devices are used has a predetermine value (usually 50 xcexa9), and the impedance at each port in the nonreciprocal circuit device (hereinafter, referred to as a xe2x80x9cport impedancexe2x80x9d) is also set to be a predetermined value.
The three center conductors which are disposed on the rectangular-parallelepiped shaped magnetic body so as to tilt toward each side of the magnetic body at an angle of 120 degrees, form a rotationally asymmetric configuration. Hence, when the conductor widths and the inter-conductor spacings of the center conductors are each made to be the same, the port impedance of the center conductor which is parallel with one side of the magnetic body becomes higher than that of each of the two other central conductors. For example, in the above-described conventional nonreciprocal circuit device, when the port impedance of the two center conductors which is disposed so as to tilt toward each side of the magnetic body is set to be 50 xcexa9, the port impedance of the conductor which is disposed parallel with one side of the magnetic body can become 80 xcexa9. This is, the two center conductors which is disposed so as to tilt toward each side of the magnetic body have a symmetric configuration with respect to the magnetic body, while the center conductor which is disposed parallel with one side of the magnetic body has a configuration asymmetric with respect to the two other center conductors. In the above-described conventional nonreciprocal circuit device, therefore, a problem occurs that the reflection characteristics of the port of the center conductor disposed parallel with one side of the magnetic body deteriorates.
Accordingly, it is an object of the present invention to provide a nonreciprocal circuit device which has an improved reflection characteristic of the port of the center conductor which is disposed parallel with one side of a magnetic body, and to provide a communication device using the same.
In order to achieve the above-described object, the present invention provides a nonreciprocal circuit device comprising a magnetic body having a rectangular parallelepiped shape, the magnetic body including three center conductors arranged to intersect one another at a predetermined angle, provided in an electrically insulated state from one another, and one of which is disposed substantially parallel with one side of the magnetic body. In this nonreciprocal circuit device, the conductor width of the center conductor which is disposed substantially parallel with one side of the magnetic body, is set to be wider than that of each of the two other center conductors. Furthermore, when each of these center conductors are constituted of a plurality of conductors, the inter-conductor spacing of the center conductor which is disposed parallel with one side of the magnetic body, is set to be wider than that of each of the two other center conductors.
In accordance with this construction, the port impedance of the center conductor which is disposed substantially parallel with one side of a magnetic body decreases, and thereby the reflection characteristics of this center conductor can be improved. Specifically, in the present invention, in order to bring the port impedance of the center conductor which is disposed parallel with one side of the magnetic body close to the port impedances of the two other center conductors, the conductor width or the inter-conductor spacing of the center conductor which is disposed parallel with one side of the magnetic body, is set to be wider than the conductor width or the inter-conductor spacing of each of the two other center conductors. This allows each port to achieve an appropriate impedance matching, which results in improved reflection characteristics at the port of each of the center conductors.
Preferably, each of the center conductors is constituted of two conductors. This allows insertion loss to be reduced by a simple structure.
Also, it is preferable that a terminating register be connected to any one of the center conductors to form an isolator. In this case, since the port of the center conductor which is disposed parallel with one side of the magnetic body has a port impedance more prone to deviate than the port impedances of the other center conductors, the port of this center conductor is suitable for an isolation port which can be terminated by a resistor having an arbitrary resistance value. It is, therefore, preferable to connect a terminating port to this port.
Moreover, a communication device in accordance with the present invention is achieved by providing it with a nonreciprocal circuit device having the above-described features. This allows a communication device having superior characteristics to be achieved.
The above and other objects, features, and advantages of the present invention will be clear from the following detailed description of the preferred embodiments of the invention in conjunction with the accompanying drawings.